1. Field of the Invention
This invention relates generally to chemical mechanical planarization apparatuses, and more particularly to methods and apparatuses for improved uniformity in chemical mechanical planarization applications via a side double roller apparatus.
2. Description of the Related Art
In the fabrication of semiconductor devices, planarization operations, which can include polishing, buffing, and wafer cleaning, are often performed. Typically, integrated circuit devices are in the form of multi-level structures. At the substrate level, transistor devices having diffusion regions are formed. In subsequent levels, interconnect metallization lines are patterned and electrically connected to the transistor devices to define the desired functional device. Patterned conductive layers are insulated from other conductive layers by dielectric materials, such as silicon dioxide. As more metallization levels and associated dielectric layers are formed, the need to planarize the dielectric material increases. Without planarization, fabrication of additional metallization layers becomes substantially more difficult due to the higher variations in the surface topography. In other applications, metallization line patterns are formed in the dielectric material, and then metal planarization operations are performed to remove excess metallization. Further applications include planarization of dielectric films deposited prior to the metallization process, such as dielectrics used for shallow trench isolation or for poly-metal insulation. One method for achieving semiconductor wafer planarization is the chemical mechanical planarization (CMP) process.
In general, the CMP process involves holding and rubbing a typically rotating wafer against a moving polishing pad under a controlled pressure and relative speed. CMP systems typically implement orbital, belt, or brush stations in which pads or brushes are used to scrub, buff, and polish one or both sides of a wafer. Slurry is used to facilitate and enhance the CMP operation. Slurry is most usually introduced onto a moving preparation surface and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface.
FIG. 1A is a diagram showing a conventional table based CMP apparatus 50. The conventional table based CMP apparatus 50 includes a polishing head 52, which holds a wafer 54, and is attached to a translation arm 64. In addition, the table based CMP apparatus 50 includes a polishing pad 56 that is disposed above a polishing table 58, which is often referred to as a polishing platen.
In operation, the polishing head 52 applies downward force to the wafer 54, which contacts the polishing pad 56. Reactive force is provided by the polishing table 58, which resists the downward force applied by the polishing head 52. The polishing pad 56 is used in conjunction with slurry to polish the wafer 54. Typically, the polishing pad 56 comprises foamed polyurethane or a sheet of polyurethane having a grooved surface. The polishing pad 56 is wetted with a polishing slurry having both an abrasive and other polishing chemicals. In addition, the polishing table 58 is rotated about its central axis 60, and the polishing head 52 is rotated about its central axis 62. Further, the polishing head can be translated across the polishing pad 56 surface using the translation arm 64. In addition to the table based CMP apparatus 50 discussed above, linear belt CMP systems have been conventionally used to perform CMP.
FIG. 1B shows a side view of a conventional linear wafer polishing apparatus 100. The linear wafer polishing apparatus 100 includes a polishing head 108, which secures and holds a wafer 104 in place during processing. A polishing pad 102 forms a continuous loop around rotating drums 112, and generally moves in a direction 106 at a speed of about 400 feet per minute, however this speed may vary depending upon the specific CMP operation. As the polishing pad 102 moves, the polishing head 108 rotates and lowers the wafer 104 onto the top surface of the polishing pad 102, loading it with required polishing pressure.
A bearing platen manifold assembly 110 supports the polishing pad 102 during the polishing process. The platen manifold assembly 110 may utilize any type of bearing such as a fluid bearing or a gas bearing. The platen manifold assembly 110 is supported and held into place by a platen surround plate 116. Gas pressure from a gas source 114 is inputted through the platen manifold assembly 110 via a plurality of independently controlled of output holes that provide upward force on the polishing pad 102 to control the polishing pad profile.
Unfortunately, in each of the above CMP systems, non-uniformities can occur in material removal rate. Generally, to achieve uniform material removal, all parameters defining the material removal rate are required to be evenly distributed across the entire contact surface that interfaces with the wafer.
Edge instabilities in CMP are among the most significant performance affecting issues and among the most complicated problems to resolve. FIG. 2 is a diagram showing a wafer pad interface 200, illustrating edge effect non-uniformity factors. As shown in FIG. 2, when the wafer 54 contacts the polishing pad 56 during the CMP process, the flexibility in the polishing pad 56 allows the wafer 54 to form a depression in the polishing pad 56. More particularly, although the polishing pad 56 is a compressible medium, the polishing pad 56 has limited flexibility, which prevents the polishing pad 56 from conforming to the exact shape of the wafer 54, forming transient deformation zones. As a result, edge effects occur at the wafer edge 202 from a non-flat contact force field resulting from redistributed contact load. Hence, large variations in removal rates occur at the wafer edge 202.
Although the air bearing platen approach utilized in a linear wafer polishing apparatus can allow significant compensation for the above mentioned non-uniformity in the CMP process, the coupling of support and pad flexing functions limits the degrees of freedom available for each function. For example, if a process engineer adjusts the air pressure to provide additional support for the wafer and polishing pad, the pressure change will also affect pad flexing, which is also being performed by the air bearing. In addition, the conventional approaches require significant air consumption to meet uniformity targets.
In view of the foregoing, there is a need for CMP systems capable of compensating for process non-uniformity. The CMP systems should be capable of compensating for non-uniformity, such as edge effect, independently of other process functions, such as wafer and pad support.
Broadly speaking, the present invention fills these needs by providing polishing pad flexing techniques that allow independent flexing of a polishing pad for resolving non-uniformity during a CMP process. In one embodiment, an apparatus for removal rate profile manipulation during a CMP process is disclosed. The apparatus includes an actuator capable of vertical movement perpendicular to a polishing surface of a polishing pad. The actuator is further capable of flexing the polishing pad independently of a pad support device. Also included in the apparatus is an actuator control mechanism that is in communication with the actuator. The actuator control mechanism is capable of controlling an amount of vertical movement of the actuator, allowing the actuator to provide local flexing of the polishing pad to achieve a particular removal rate profile. The actuator can also be capable of horizontal movement parallel to the polishing surface of the polishing pad. In one aspect, the actuator can be a double roller that comprises a first roller above the polishing pad and a second roller below the polishing pad, allowing the polishing pad to be flexed toward a wafer being planarized and away from the wafer being planarized. In a further aspect, the actuator can be a double slider that comprises a first slider above the polishing pad and a second slider below the polishing pad, allowing the polishing pad to be flexed toward a wafer being planarized and away from the wafer being planarized. In one aspect, each slider can project a liquid or gas toward the polishing pad to reduce friction.
In a further embodiment, a method is disclosed for manipulating a removal rate profile during a CMP process. An actuator is provided that is capable of vertical movement perpendicular to a polishing surface of a polishing pad. As above, the actuator is capable of flexing the polishing pad independently of a pad support device. The vertical position of the actuator relative to the polishing pad is then altered to locally flex the polishing pad to achieve a particular removal rate profile. Optionally, the horizontal position of the actuator parallel to the polishing surface of the polishing pad can be altered to further locally flex the polishing pad to achieve a particular removal rate profile.
A system for removal rate profile manipulation during a CMP process is disclosed in a further embodiment of the present invention. The system includes a polishing pad comprising a flexible material that is capable of planarizing a wafer. Below the polishing pad is a pad support device that is capable of providing reactive force to the wafer during a CMP process. For example, the pad support device can be a polishing table or an air bearing. The system further includes an actuator that is capable of vertical movement perpendicular to a polishing surface of the polishing pad and horizontal movement parallel to the polishing pad. Further, the actuator is capable of flexing the polishing pad independently of the pad support device. In communication with the actuator is an actuator control mechanism. The actuator control mechanism is capable of controlling the amount of vertical and horizontal movement of the actuator, such that the actuator provides local flexing of the polishing pad to achieve a particular removal rate profile. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.